On This Day . . .

Math Finds Furthest Planet

September 23, 1846: Eighth Planet Discovered

German astronomer Johann Gottfried Galle discovers the planet Neptune at the Berlin Observatory.

Johann Galle

Urbain Le Verrier

Neptune was the first planet found by mathematical prediction rather than by observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed on 23 September 1846 by Johann Galle within a degree of the position also predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining 13 moons were located telescopically until the 20th century

Le Verrier by letter, urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory’s refractor. Heinrich d’Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier’s predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star.

The very evening of the day of receipt of Le Verrier’s letter on 23 September 1846, Neptune was discovered within 1° of where Le Verrier had predicted it to be.

Earth compared to Neptune

Light Blue object represents Uranus

The blue gas giant has a diameter four times that of Earth. It has 13 known moons, of which Triton is the largest, and a ring system containing three bright and two dim rings. It completes an orbit of the sun once every 165 years. In 1989, the U.S. planetary spacecraft Voyager 2 was the first human spacecraft to visit Neptune.

Voyager 2 sent back this stunning image of storms at work in Neptune’s windy atmosphere in August 1989

This Voyager 2 high resolution color image, taken 2 hours before closest approach, provides obvious evidence of vertical relief in Neptune’s bright cloud streaks.These clouds were observed near Neptune’s east terminator. The linear cloud forms are stretched approximately along lines of constant latitude and the Sun is toward the lower left. The bright sides of the clouds which face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun.

Neptune is the fourth-largest planet by diameter and the third-largest by mass. Among the gaseous planets in the solar system, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth–Sun distance. Its astronomical symbol is ♆, a stylized version of the god Neptune’s trident.

Shortly after its discovery, Neptune was referred to simply as “the planet exterior to Uranus” or as “Le Verrier’s planet”. Claiming the right to name his discovery, Le Verrier quickly proposed the name Neptune for this new planet, while falsely stating that this had been officially approved by the French Bureau des Longitudes. Soon Neptune became the internationally accepted name. In Roman mythology, Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Earth, were named for Greek and Roman mythology.

10 Facts About Neptune

1. Neptune is the most distant planet

2. Neptune is the smallest of the gas giants
With an equatorial radius of only 24,764 km, Neptune is smaller than the other gas giants in the Solar System: Jupiter, Saturn and Uranus. But here’s the funny thing, Neptune is actually more massive than Uranus by about 18%. Since it’s smaller but more massive, Neptune is much more dense than Uranus.

3. Neptune’s surface gravity is almost Earthlike

Neptune is a ball of gas and ice, probably with a rocky core. There’s no way you could actually stand on the surface of Neptune without just sinking in. However, if you could stand on the surface of Neptune, you would notice something amazing. The force of gravity pulling you down is almost exactly the same as the force of gravity you feel walking here on Earth.

4. The discovery of Neptune is still a controversy
The first person to probably see Neptune was Galileo, who marked it as a star in one of his drawings. He didn’t realize what he was looking at, so that doesn’t count. The French mathematician Urbain Le Verrier and the English mathematician John Couch Adams both made predictions that a new planet would be discovered in a specific region of the sky.

When astronomer Johann Gottfried Galle actually found the planet in 1846, both mathematicians took credit for the discovery. English and French astronomers battled over who actually made the discovery, and there are still defenders of each claim to this day. Most astronomers consider that Le Verrier and Adams shared the discovery, and don’t worry about it anymore.

5. Neptune has the strongest winds in the Solar System
Think a hurricane is scary? Imagine a hurricane with winds that go up to 2,100 km/hour.

6. Neptune is the coldest planet in the Solar System
At the top of its clouds, temperatures on Neptune can dip down to 51.7 Kelvin, or -221.4 degrees Celsius

7. Neptune’s moon Triton is even colder
There are many cold places in the Solar System, but one of the coldest is the surface of Neptune’s moon Triton. This is the largest of Neptune’s 13 moons, and the only one with enough mass and gravity to pull itself into a sphere. In fact, it’s the 7th largest moon in the Solar System. Temperatures on the surface of Triton can dip down to only 38 Kelvin or – 235 degrees Celsius. But even though it’s incredibly cold, the surface of Triton is very active. During its 1989 flyby, NASA’s Voyager 2 spacecraft observed volcanoes or geysers erupting liquid nitrogen.

Triton

8. Neptune probably captured Triton
Neptune’s largest Moon, Triton, orbits in a retrograde orbit, that means that it orbits around the planet backwards compared to the other moons that orbit Neptune. This means that Neptune probably captured Triton; the moon didn’t form in place like the rest of Neptune’s moons. Triton is locked into a synchronous rotation with Neptune and is slowly spiraling inward towards the planet. At some point, billions of years from now, it’ll be torn apart by Neptune’s gravitational forces and become a magnificent ring around the planet. Afterward the ring will be pulled inward to crash into the planet. It would be amazing to watch.

Voyager II mosaic of Triton

9. Neptune has only been visited once up close
The only spacecraft that has ever visited Neptune was NASA’s Voyager 2 spacecraft, which visited the planet during its Grand Tour of the Solar System. Voyager 2 made its Neptune flyby on August 25, 1989, passing within 3,000 km of the planet’s north pole. This was the closest approach to any object that Voyager 2 made since it was launched from Earth. During its flyby, Voyager 2 studied Neptune’s atmosphere, its rings, magnetosphere. It also made observations of Neptune’s moons.

10. There are no plans to visit Neptune again
Voyager 2′s amazing photographs of Neptune might be all we get for decades. There are no firm plans to return to Neptune. There are tentative plans from NASA to send a new mission to Neptune called the Neptune Orbiter. This spacecraft would launch in 2016 and take about 14 years to get to Neptune, arriving around 2030. It would go into orbit around the planet and study its weather, magnetosphere, ring system and moons.